Redundant, fault-tolerant, distributed remote procedure call cache in a storage system

ABSTRACT

A method of operating a remote procedure call cache in a storage cluster is provided. The method includes receiving a remote procedure call at a first storage node having solid-state memory and writing information, relating to the remote procedure call, to a remote procedure call cache of the first storage node. The method includes mirroring the remote procedure call cache of the first storage node in a mirrored remote procedure call cache of a second storage node. A plurality of storage nodes and a storage cluster are also provided.

BACKGROUND

Solid-state memory, such as flash, is currently in use in solid-statedrives (SSD) to augment or replace conventional hard disk drives (HDD),writable CD (compact disk) or writable DVD (digital versatile disk)drives, collectively known as spinning media, and tape drives, forstorage of large amounts of data. Flash and other solid-state memorieshave characteristics that differ from spinning media. Yet, manysolid-state drives are designed to conform to hard disk drive standardsfor compatibility reasons, which makes it difficult to provide enhancedfeatures or take advantage of unique aspects of flash and othersolid-state memory. Solid-state drives, and other types of storage, maybe vulnerable to corruption if a failure occurs during servicing of aremote procedure call (RPC).

It is within this context that the embodiments arise.

SUMMARY

In some embodiments, a method of operating a remote procedure call cachein a storage cluster is provided. The method includes receiving a remoteprocedure call at a first storage node having solid-state memory andwriting information, relating to the remote procedure call, to a remoteprocedure call cache of the first storage node. The method includesmirroring the remote procedure call cache of the first storage node in amirrored remote procedure call cache of a second storage node. Aplurality of storage nodes and a storage cluster are also provided.

Other aspects and advantages of the embodiments will become apparentfrom the following detailed description taken in conjunction with theaccompanying drawings which illustrate, by way of example, theprinciples of the described embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The described embodiments and the advantages thereof may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings. These drawings in no waylimit any changes in form and detail that may be made to the describedembodiments by one skilled in the art without departing from the spiritand scope of the described embodiments.

FIG. 1 is a perspective view of a storage cluster with multiple storagenodes and internal storage coupled to each storage node to providenetwork attached storage, in accordance with some embodiments.

FIG. 2 is a block diagram showing a communications interconnect couplingmultiple storage nodes in accordance with some embodiments.

FIG. 3 is a multiple level block diagram, showing contents of a storagenode and contents of one of the non-volatile solid-state storage unitsin accordance with some embodiments.

FIG. 4 is a block diagram showing remote procedure calls in a storagesystem in accordance with some embodiments.

FIG. 5 is a block diagram of storage nodes with redundant remoteprocedure call caches in accordance with some embodiments.

FIG. 6 is a flow diagram of a method for caching a remote procedure callin a storage cluster, which can be practiced on or by embodiments of thestorage cluster, storage nodes and/or solid-state storages disclosedherein in accordance with some embodiments.

FIG. 7 is an illustration showing an exemplary computing device whichmay implement the embodiments described herein.

DETAILED DESCRIPTION

The embodiments below describe a storage cluster that stores user data,such as user data originating from one or more user or client systems orother sources external to the storage cluster. The storage clusterdistributes user data across storage nodes housed within a chassis,using erasure coding and redundant copies of metadata. Erasure codingrefers to a method of data protection in which data is broken intofragments, expanded and encoded with redundant data pieces and storedacross a set of different locations, such as disks, storage nodes orgeographic locations. Flash memory is one type of solid-state memorythat may be integrated with the embodiments, although the embodimentsmay be extended to other types of solid-state memory or other storagemedium, including non-solid state memory. Control of storage locationsand workloads are distributed across the storage locations in aclustered peer-to-peer system. Tasks such as mediating communicationsbetween the various storage nodes, detecting when a storage node hasbecome unavailable, and balancing I/Os (inputs and outputs) across thevarious storage nodes, are all handled on a distributed basis. Data islaid out or distributed across multiple storage nodes in data fragmentsor stripes that support data recovery in some embodiments. Ownership ofdata can be reassigned within a cluster, independent of input and outputpatterns. This architecture described in more detail below allows astorage node in the cluster to fail, with the system remainingoperational, since the data can be reconstructed from other storagenodes and thus remain available for input and output operations. Invarious embodiments, a storage node may be referred to as a clusternode, a blade, or a server.

The storage cluster is contained within a chassis, i.e., an enclosurehousing one or more storage nodes. A mechanism to provide power to eachstorage node, such as a power distribution bus, and a communicationmechanism, such as a communication bus that enables communicationbetween the storage nodes are included within the chassis. The storagecluster can run as an independent system in one location according tosome embodiments. In one embodiment, a chassis contains at least twoinstances of both the power distribution and the communication bus whichmay be enabled or disabled independently. The internal communication busmay be an Ethernet bus, however, other technologies such as PeripheralComponent Interconnect (PCI) Express, InfiniBand, and others, areequally suitable. The chassis provides a port for an externalcommunication bus for enabling communication between multiple chassis,directly or through a switch, and with client systems. The externalcommunication may use a technology such as Ethernet, InfiniBand, FibreChannel, etc. In some embodiments, the external communication bus usesdifferent communication bus technologies for inter-chassis and clientcommunication. If a switch is deployed within or between chassis, theswitch may act as a translation between multiple protocols ortechnologies. When multiple chassis are connected to define a storagecluster, the storage cluster may be accessed by a client using eitherproprietary interfaces or standard interfaces such as network filesystem (NFS), common internet file system (CIFS), small computer systeminterface (SCSI) or hypertext transfer protocol (HTTP). Translation fromthe client protocol may occur at the switch, chassis externalcommunication bus or within each storage node.

Each storage node may be one or more storage servers and each storageserver is connected to one or more non-volatile solid-state memoryunits, which may be referred to as storage units. One embodimentincludes a single storage server in each storage node and between one toeight non-volatile solid-state memory units, however this one example isnot meant to be limiting. The storage server may include a processor,dynamic random access memory (DRAM) and interfaces for the internalcommunication bus and power distribution for each of the power buses.Inside the storage node, the interfaces and storage unit share acommunication bus, e.g., PCI Express, in some embodiments. Thenon-volatile solid-state memory units may directly access the internalcommunication bus interface through a storage node communication bus, orrequest the storage node to access the bus interface. The non-volatilesolid-state memory unit contains an embedded central processing unit(CPU), solid-state storage controller, and a quantity of solid-statemass storage, e.g., between 2-32 terabytes (TB) in some embodiments. Anembedded volatile storage medium, such as DRAM, and an energy reserveapparatus are included in the non-volatile solid-state memory unit. Insome embodiments, the energy reserve apparatus is a capacitor,super-capacitor, or battery that enables transferring a subset of DRAMcontents to a stable storage medium in the case of power loss. In someembodiments, the non-volatile solid-state memory unit is constructedwith a storage class memory, such as phase change or magnetoresistiverandom access memory (MRAM) that substitutes for DRAM and enables areduced power hold-up apparatus.

One of many features of the storage clusters, storage nodes andnon-volatile solid-state storages disclosed herein is a redundant,fault-tolerant, distributed remote procedure call cache. This featureprevents corruption that can occur when a failure disrupts a remoteprocedure call in progress. By storing multiple copies of a remoteprocedure call cache across the storage nodes, a storage cluster is ableto continue operating despite a failure causing disruption of a remoteprocedure call service, loss of contents of or access to a remoteprocedure call cache, or loss of other resource(s) involved in servicingthe remote procedure call. A remote procedure call that is repeated(e.g., by a client or host system) is serviced only once internally tothe storage cluster, but the result is returned each time the remoteprocedure call is repeated. Fault-tolerant support for multiplefilesystems is hereby provided. These and further details of variousstorage systems and operation thereof are discussed below.

FIG. 1 is a perspective view of a storage cluster 160, with multiplestorage nodes 150 and internal solid-state memory coupled to eachstorage node to provide network attached storage or storage areanetwork, in accordance with some embodiments. A network attachedstorage, storage area network, or a storage cluster, or other storagememory, could include one or more storage clusters 160, each having oneor more storage nodes 150, in a flexible and reconfigurable arrangementof both the physical components and the amount of storage memoryprovided thereby. The storage cluster 160 is designed to fit in a rack,and one or more racks can be set up and populated as desired for thestorage memory. The storage cluster 160 has a chassis 138 havingmultiple slots 142. It should be appreciated that chassis 138 may bereferred to as a housing, enclosure, or rack unit. In one embodiment,the chassis 138 has fourteen slots 142, although other numbers of slotsare readily devised. For example, some embodiments have four slots,eight slots, sixteen slots, thirty-two slots, or other suitable numberof slots. Each slot 142 can accommodate one storage node 150 in someembodiments. Chassis 138 includes flaps 148 that can be utilized tomount the chassis 138 on a rack. Fans 144 provide air circulation forcooling of the storage nodes 150 and components thereof, although othercooling components could be used, or an embodiment could be devisedwithout cooling components. A switch fabric 146 couples storage nodes150 within chassis 138 together and to a network for communication tothe memory. In an embodiment depicted in FIG. 1, the slots 142 to theleft of the switch fabric 146 and fans 144 are shown occupied by storagenodes 150, while the slots 142 to the right of the switch fabric 146 andfans 144 are empty and available for insertion of storage node 150 forillustrative purposes. This configuration is one example, and one ormore storage nodes 150 could occupy the slots 142 in various furtherarrangements. The storage node arrangements need not be sequential oradjacent in some embodiments. Storage nodes 150 are hot pluggable,meaning that a storage node 150 can be inserted into a slot 142 in thechassis 138, or removed from a slot 142, without stopping or poweringdown the system. Upon insertion or removal of storage node 150 from slot142, the system automatically reconfigures in order to recognize andadapt to the change. Reconfiguration, in some embodiments, includesrestoring redundancy and/or rebalancing data or load.

Each storage node 150 can have multiple components. In the embodimentshown here, the storage node 150 includes a printed circuit board 158populated by a CPU 156, i.e., processor, a memory 154 coupled to the CPU156, and a non-volatile solid-state storage 152 coupled to the CPU 156,although other mountings and/or components could be used in furtherembodiments. The memory 154 has instructions which are executed by theCPU 156 and/or data operated on by the CPU 156. As further explainedbelow, the non-volatile solid-state storage 152 includes flash or, infurther embodiments, other types of solid-state memory.

Storage cluster 160 is scalable, meaning that storage capacity withnon-uniform storage sizes is readily added, as described above. One ormore storage nodes 150 can be plugged into or removed from each chassisand the storage cluster self-configures in some embodiments. Plug-instorage nodes 150, whether installed in a chassis as delivered or lateradded, can have different sizes. For example, in one embodiment astorage node 150 can have any multiple of 4 TB, e.g., 8 TB, 12 TB, 16TB, 32 TB, etc. In further embodiments, a storage node 150 could haveany multiple of other storage amounts or capacities. Storage capacity ofeach storage node 150 is broadcast, and influences decisions of how tostripe the data. For maximum storage efficiency, an embodiment canself-configure as wide as possible in the stripe, subject to apredetermined requirement of continued operation with loss of up to one,or up to two, non-volatile solid-state storage units 152 or storagenodes 150 within the chassis.

FIG. 2 is a block diagram showing a communications interconnect 170 andpower distribution bus 172 coupling multiple storage nodes 150.Referring back to FIG. 1, the communications interconnect 170 can beincluded in or implemented with the switch fabric 146 in someembodiments. Where multiple storage clusters 160 occupy a rack, thecommunications interconnect 170 can be included in or implemented with atop of rack switch, in some embodiments. As illustrated in FIG. 2,storage cluster 160 is enclosed within a single chassis 138. Externalport 176 is coupled to storage nodes 150 through communicationsinterconnect 170, while external port 174 is coupled directly to astorage node. External power port 178 is coupled to power distributionbus 172. Storage nodes 150 may include varying amounts and differingcapacities of non-volatile solid-state storage 152. In addition, one ormore storage nodes 150 may be a compute only storage node. Authorities168 are implemented on the non-volatile solid-state storages 152, forexample as lists or other data structures stored in memory. In someembodiments the authorities are stored within the non-volatilesolid-state storage 152 and supported by software executing on acontroller or other processor of the non-volatile solid-state storage152. In a further embodiment, authorities 168 are implemented on thestorage nodes 150, for example as lists or other data structures storedin the memory 154 and supported by software executing on the CPU 156 ofthe storage node 150. Authorities 168, which can be viewed as roles thatthe storage nodes 150 take on, control how and where data is stored inthe non-volatile solid-state storages 152 in some embodiments. Thiscontrol assists in determining which type of erasure coding scheme isapplied to the data, and which storage nodes 150 have which portions ofthe data. Each authority 168 may be assigned to a non-volatilesolid-state storage 152. Each authority may control a range of inodenumbers, segment numbers, or other data identifiers which are assignedto data by a file system, by the storage nodes 150, or by thenon-volatile solid-state storage 152, in various embodiments.

Every piece of data, and every piece of metadata, has redundancy in thesystem in some embodiments. In addition, every piece of data and everypiece of metadata has an owner, which may be referred to as anauthority. If that authority is unreachable, for example through failureof a storage node, there is a plan of succession for how to find thatdata or that metadata. In various embodiments, there are redundantcopies of authorities 168. Authorities 168 have a relationship tostorage nodes 150 and non-volatile solid-state storage 152 in someembodiments. Each authority 168, covering a range of data segmentnumbers or other identifiers of the data, may be assigned to a specificnon-volatile solid-state storage 152. In some embodiments theauthorities 168 for all of such ranges are distributed over thenon-volatile solid-state storages 152 of a storage cluster. Each storagenode 150 has a network port that provides access to the non-volatilesolid-state storage(s) 152 of that storage node 150. Data can be storedin a segment, which is associated with a segment number and that segmentnumber is an indirection for a configuration of a RAID (redundant arrayof independent disks) stripe in some embodiments. The assignment and useof the authorities 168 thus establishes an indirection to data.Indirection may be referred to as the ability to reference dataindirectly, in this case via an authority 168, in accordance with someembodiments. A segment identifies a set of non-volatile solid-statestorage 152 and a local identifier into the set of non-volatilesolid-state storage 152 that may contain data. In some embodiments, thelocal identifier is an offset into the device and may be reusedsequentially by multiple segments. In other embodiments the localidentifier is unique for a specific segment and never reused. Theoffsets in the non-volatile solid-state storage 152 are applied tolocating data for writing to or reading from the non-volatilesolid-state storage 152 (in the form of a RAID stripe). Data is stripedacross multiple units of non-volatile solid-state storage 152, which mayinclude or be different from the non-volatile solid-state storage 152having the authority 168 for a particular data segment.

If there is a change in where a particular segment of data is located,e.g., during a data move or a data reconstruction, the authority 168 forthat data segment should be consulted, at that non-volatile solid-statestorage 152 or storage node 150 having that authority 168. In order tolocate a particular piece of data, embodiments calculate a hash valuefor a data segment or apply an inode number or a data segment number.The output of this operation points to a non-volatile solid-statestorage 152 having the authority 168 for that particular piece of data.This non-volatile solid-state storage 152, as the authority owner forthe data segment, can coordinate a move or reconstruction of the data.In some embodiments there are two stages to this operation. The firststage maps an entity identifier (ID), e.g., a segment number, inodenumber, or directory number to an authority identifier. This mapping mayinclude a calculation such as a hash or a bit mask. The second stage ismapping the authority identifier to a particular non-volatilesolid-state storage 152, which may be done through an explicit mapping.The operation is repeatable, so that when the calculation is performed,the result of the calculation repeatably and reliably points to aparticular non-volatile solid-state storage 152 having that authority168. The operation may include the set of reachable storage nodes asinput. If the set of reachable non-volatile solid-state storage unitschanges the optimal set changes. In some embodiments, the persistedvalue (i.e., the value that persistently results from the calculation)is the current assignment (which is always true) and the calculatedvalue is the target assignment the cluster will attempt to reconfiguretowards. This calculation may be used to determine the optimalnon-volatile solid-state storage 152 for an authority in the presence ofa set of non-volatile solid-state storage 152 that are reachable andconstitute the same cluster. The calculation also determines an orderedset of peer non-volatile solid-state storages 152 that will also recordthe authority to non-volatile solid-state storage mapping so that theauthority may be determined even if the assigned non-volatilesolid-state storage is unreachable. A duplicate or substitute authority168 may be consulted if a specific authority 168 is unavailable in someembodiments.

Two of the many tasks of the CPU 156 on a storage node 150 are to breakup write data, and reassemble read data. When the system has determinedthat data is to be written, the authority 168 for that data is locatedas above. When the segment ID for data is-determined, the request towrite is forwarded to the non-volatile solid-state storage 152 currentlydetermined to be the host of the authority 168 determined from thesegment. The host CPU 156 of the storage node 150, on which thenon-volatile solid-state storage 152 and corresponding authority 168reside, then breaks up or shards the data and transmits the data out tovarious non-volatile solid-state storages 152. In some embodiments, theauthority 168 for the data segment being written to may defer shardingand distributing data to be done asynchronously after establishingredundancy for that data itself. The transmitted data is written as adata stripe in accordance with an erasure coding scheme. In someembodiments, data is requested to be pulled, and in other embodiments,data is pushed. In reverse, when data is read, the authority 168 for thesegment ID containing the data is located as described above. The hostCPU 156 of the storage node 150 on which the non-volatile solid-statestorage 152 and corresponding authority 168 reside requests the datafrom the non-volatile solid-state storage and corresponding storagenodes pointed to by the authority. In some embodiments the data is readfrom flash storage as a data stripe. The host CPU 156 of storage node150 then reassembles the read data, correcting any errors (if present)according to the appropriate erasure coding scheme, and forwards thereassembled data to the network. In further embodiments, some or all ofthese tasks can be handled in the non-volatile solid-state storage 152.In some embodiments, the segment host requests the data be sent tostorage node 150 by requesting pages from storage and then sending thedata to the storage node 150 making the original request. In someembodiments, a stripe width is only read if there is a single page readfailure or delay.

In some systems, for example in UNIX-style file systems, data is handledwith an index node or inode, which specifies a data structure thatrepresents an object in a file system. The object could be a file or adirectory, for example. Metadata may accompany the object, as attributessuch as permission data and a creation timestamp, among otherattributes. A segment number could be assigned to all or a portion ofsuch an object in a file system. In other systems, data segments arehandled with a segment number assigned elsewhere. For purposes ofdiscussion, the unit of distribution is an entity, and an entity can bea file, a directory or a segment. That is, entities are units of data ormetadata stored by a storage system. Entities are grouped into setscalled authorities. Each authority has an authority owner, which is astorage node that has the exclusive right to update the entities in theauthority. In other words, a storage node contains the authority, andthat the authority, in turn, contains entities.

A segment is a logical container of data in accordance with someembodiments. A segment is an address space between medium address spaceand physical flash locations, i.e., the data segment number, are in thisaddress space. Segments may also contain metadata, which enable dataredundancy to be restored (rewritten to different flash locations ordevices) without the involvement of higher level software. In oneembodiment, an internal format of a segment contains client data andmedium mappings to determine the position of that data. Each datasegment is protected, e.g., from memory and other failures, by breakingthe segment into a number of data and parity shards, where applicable.The data and parity shards are distributed, i.e., striped, acrossnon-volatile solid-state storage 152 coupled to the host CPUs 156 inaccordance with an erasure coding scheme. Usage of the term segmentsrefers to the container and its place in the address space of segmentsin some embodiments. Usage of the term stripe refers to the same set ofshards as a segment and includes how the shards are distributed alongwith redundancy or parity information in accordance with someembodiments.

A series of address-space transformations takes place across an entirestorage system. At the top are the directory entries (file names) whichlink to an inode. Inodes point into medium address space, where data islogically stored. Medium addresses may be mapped through a series ofindirect mediums to spread the load of large files, or implement dataservices like deduplication or snapshots. Medium addresses may be mappedthrough a series of indirect mediums to spread the load of large files,or implement data services like deduplication or snapshots. Segmentaddresses are then translated into physical flash locations. Physicalflash locations have an address range bounded by the amount of flash inthe system in accordance with some embodiments. Medium addresses andsegment addresses are logical containers, and in some embodiments use a128 bit or larger identifier so as to be practically infinite, with alikelihood of reuse calculated as longer than the expected life of thesystem. Addresses from logical containers are allocated in ahierarchical fashion in some embodiments. Initially, each non-volatilesolid-state storage 152 may be assigned a range of address space. Withinthis assigned range, the non-volatile solid-state storage 152 is able toallocate addresses without synchronization with other non-volatilesolid-state storage 152.

Data and metadata is stored by a set of underlying storage layouts thatare optimized for varying workload patterns and storage devices. Theselayouts incorporate multiple redundancy schemes, compression formats andindex algorithms. Some of these layouts store information aboutauthorities and authority masters, while others store file metadata andfile data. The redundancy schemes include error correction codes thattolerate corrupted bits within a single storage device (such as a NANDflash chip), erasure codes that tolerate the failure of multiple storagenodes, and replication schemes that tolerate data center or regionalfailures. In some embodiments, low density parity check (LDPC) code isused within a single storage unit. Reed-Solomon encoding is used withina storage cluster, and mirroring is used within a storage grid in someembodiments. Metadata may be stored using an ordered log structuredindex (such as a Log Structured Merge Tree), and large data may not bestored in a log structured layout.

In order to maintain consistency across multiple copies of an entity,the storage nodes agree implicitly on two things through calculations:(1) the authority that contains the entity, and (2) the storage nodethat contains the authority. The assignment of entities to authoritiescan be done by pseudorandomly assigning entities to authorities, bysplitting entities into ranges based upon an externally produced key, orby placing a single entity into each authority. Examples of pseudorandomschemes are linear hashing and the Replication Under Scalable Hashing(RUSH) family of hashes, including Controlled Replication Under ScalableHashing (CRUSH). In some embodiments, pseudo-random assignment isutilized only for assigning authorities to nodes because the set ofnodes can change. The set of authorities cannot change so any subjectivefunction may be applied in these embodiments. Some placement schemesautomatically place authorities on storage nodes, while other placementschemes rely on an explicit mapping of authorities to storage nodes. Insome embodiments, a pseudorandom scheme is utilized to map from eachauthority to a set of candidate authority owners. A pseudorandom datadistribution function related to CRUSH may assign authorities to storagenodes and create a list of where the authorities are assigned. Eachstorage node has a copy of the pseudorandom data distribution function,and can arrive at the same calculation for distributing, and laterfinding or locating an authority. Each of the pseudorandom schemesrequires the reachable set of storage nodes as input in some embodimentsin order to conclude the same target nodes. Once an entity has beenplaced in an authority, the entity may be stored on physical devices sothat no expected failure will lead to unexpected data loss. In someembodiments, rebalancing algorithms attempt to store the copies of allentities within an authority in the same layout and on the same set ofmachines.

Examples of expected failures include device failures, stolen machines,datacenter fires, and regional disasters, such as nuclear or geologicalevents. Different failures lead to different levels of acceptable dataloss. In some embodiments, a stolen storage node impacts neither thesecurity nor the reliability of the system, while depending on systemconfiguration, a regional event could lead to no loss of data, a fewseconds or minutes of lost updates, or even complete data loss.

In the embodiments, the placement of data for storage redundancy isindependent of the placement of authorities for data consistency. Insome embodiments, storage nodes that contain authorities do not containany persistent storage. Instead, the storage nodes are connected tonon-volatile solid-state storage units that do not contain authorities.The communications interconnect between storage nodes and non-volatilesolid-state storage units consists of multiple communicationtechnologies and has non-uniform performance and fault tolerancecharacteristics. In some embodiments, as mentioned above, non-volatilesolid-state storage units are connected to storage nodes via PCIexpress, storage nodes are connected together within a single chassisusing an Ethernet backplane, and chassis are connected together to forma storage cluster. Storage clusters are connected to clients usingEthernet or fiber channel in some embodiments. If multiple storageclusters are configured into a storage grid, the multiple storageclusters are connected using the Internet or other long-distancenetworking links, such as a “metro scale” link or private link that doesnot traverse the internet.

Authority owners have the exclusive right to modify entities, to migrateentities from one non-volatile solid-state storage unit to anothernon-volatile solid-state storage unit, and to add and remove copies ofentities. This allows for maintaining the redundancy of the underlyingdata. When an authority owner fails, is going to be decommissioned, oris overloaded, the authority is transferred to a new storage node.Transient failures make it non-trivial to ensure that all non-faultymachines agree upon the new authority location. The ambiguity thatarises due to transient failures can be achieved automatically by aconsensus protocol such as Paxos, hot-warm failover schemes, via manualintervention by a remote system administrator, or by a local hardwareadministrator (such as by physically removing the failed machine fromthe cluster, or pressing a button on the failed machine). In someembodiments, a consensus protocol is used, and failover is automatic. Iftoo many failures or replication events occur in too short a timeperiod, the system goes into a self-preservation mode and haltsreplication and data movement activities until an administratorintervenes in accordance with some embodiments.

As authorities are transferred between storage nodes and authorityowners update entities in their authorities, the system transfersmessages between the storage nodes and non-volatile solid-state storageunits. With regard to persistent messages, messages that have differentpurposes are of different types. Depending on the type of the message,the system maintains different ordering and durability guarantees. Asthe persistent messages are being processed, the messages aretemporarily stored in multiple durable and non-durable storage hardwaretechnologies. In some embodiments, messages are stored in RAM, NVRAM andon NAND flash devices, and a variety of protocols are used in order tomake efficient use of each storage medium. Latency-sensitive clientrequests may be persisted in replicated NVRAM, and then later NAND,while background rebalancing operations are persisted directly to NAND.

Persistent messages are persistently stored prior to being replicated.This allows the system to continue to serve client requests despitefailures and component replacement. Although many hardware componentscontain unique identifiers that are visible to system administrators,manufacturers, hardware supply chains and ongoing monitoring qualitycontrol infrastructure, applications running on top of theinfrastructure can address virtualized addresses. These virtualizedaddresses do not change over the lifetime of the storage system,regardless of component failures and replacements. This allows eachcomponent of the storage system to be replaced over time withoutreconfiguration or disruptions of client request processing.

In some embodiments, the virtualized addresses are stored withsufficient redundancy. A continuous monitoring system correlateshardware and software status and the hardware identifiers. This allowsdetection and prediction of failures due to faulty components andmanufacturing details. The monitoring system also enables the proactivetransfer of authorities and entities away from impacted devices beforefailure occurs by removing the component from the critical path in someembodiments.

In addition to component redundancy in the communication channel,storage cluster 160 is configured to allow for the loss of one or morestorage nodes 150. In some embodiments this cluster redundancy level maybe one for relatively small storage clusters 160 (less than 8 storagenodes 150) and two for relatively larger storage clusters 160 (8 or morestorage nodes 150) although any number would be suitable for the clusterredundancy level. In some embodiments, where more storage nodes 150 thanthe redundancy level are lost, the storage cluster 160 cannot guaranteeavailability of data or integrity of future updates. As mentioned above,data redundancy is implemented via segments. A segment is formed byselecting equal sized shards from a subset of the non-volatilesolid-state storage 152, each within a different storage node 150.Shards are reserved to establish the redundancy level, e.g., one or two,and then a remainder constitutes the data (the data shards). The shardsare encoded using an ECC scheme such as parity or Reed-Soloman (RAID 6),so that any subset of the shards equal in count to the data shards maybe used to reconstruct the complete data. The storage cluster redundancyrepresents a minimum level of redundancy and it may be exceeded for anyindividual data element. Segments are stored as a set of non-volatilesolid-state storage units, roles (data position or parity) andallocation unit local to each non-volatile solid-state storage unit. Theallocation units may be a physical address or an indirection determinedwithin the non-volatile solid-state storage 152. Each shard may beportioned into pages and each page into code words. In some embodiments,the pages are between about 4 kilobytes (kB) and 64 kB, e.g., 16 kB,while the code words are between about 512 bytes to 4 kB, e.g., 1 kB.These sizes are one example and not meant to be limiting as any suitablesize for the code words and the pages may be utilized. The code wordscontain local error correction and a checksum to verify the errorcorrection was successful. This checksum is “salted” with the logicaladdress of the contents meaning that a failure to match the checksum mayoccur if the data is uncorrectable or misplaced. In some embodiments,when a code word fails a checksum it is converted to an “erasure” forpurpose of the error correction algorithm so that the code word may berebuilt.

FIG. 3 is a multiple level block diagram, showing contents of a storagenode 150 and contents of a non-volatile solid-state storage 152 of thestorage node 150. Data is communicated to and from the storage node 150by a network interface controller (NIC) 202 in some embodiments. Eachstorage node 150 has a CPU 156, and one or more non-volatile solid-statestorage 152, as discussed above. Moving down one level in FIG. 3, eachnon-volatile solid-state storage 152 has a relatively fast non-volatilesolid-state memory, such as non-volatile random access memory (NVRAM)204, and flash memory 206. In some embodiments, NVRAM 204 may be acomponent that does not require program/erase cycles (DRAM, MRAM, PCM),and can be a memory that can support being written vastly more oftenthan the memory is read from. Moving down another level in FIG. 3, theNVRAM 204 is implemented in one embodiment as high speed volatilememory, such as dynamic random access memory (DRAM) 216, backed up byenergy reserve 218. Energy reserve 218 provides sufficient electricalpower to keep the DRAM 216 powered long enough for contents to betransferred to the flash memory 206 in the event of power failure. Insome embodiments, energy reserve 218 is a capacitor, super-capacitor,battery, or other device, that supplies a suitable supply of energysufficient to enable the transfer of the contents of DRAM 216 to astable storage medium in the case of power loss. The flash memory 206 isimplemented as multiple flash dies 222, which may be referred to aspackages of flash dies 222 or an array of flash dies 222. It should beappreciated that the flash dies 222 could be packaged in any number ofways, with a single die per package, multiple dies per package (i.e.multichip packages), in hybrid packages, as bare dies on a printedcircuit board or other substrate, as encapsulated dies, etc. In theembodiment shown, the non-volatile solid-state storage 152 has acontroller 212 or other processor, and an input output (I/O) port 210coupled to the controller 212. I/O port 210 is coupled to the CPU 156and/or the network interface controller 202 of the flash storage node150. Flash input output (I/O) port 220 is coupled to the flash dies 222,and a direct memory access unit (DMA) 214 is coupled to the controller212, the DRAM 216 and the flash dies 222. In the embodiment shown, theI/O port 210, controller 212, DMA unit 214 and flash I/O port 220 areimplemented on a programmable logic device (PLD) 208, e.g., a fieldprogrammable gate array (FPGA). In this embodiment, each flash die 222has pages, organized as sixteen kB (kilobyte) pages 224, and a register226 through which data can be written to or read from the flash die 222.In further embodiments, other types of solid-state memory are used inplace of, or in addition to flash memory illustrated within flash die222.

FIG. 4 is a block diagram showing remote procedure calls in a storagesystem in accordance with some embodiments. A remote procedure call canbe initiated by a client, and executed on one of the storage nodes inthe storage system. Some embodiments of the storage nodes 150 haveredundant remote procedure call caches which offer fault tolerance thatprotects the system against loss of a remote procedure call in event ofa failure. Some embodiments of the storage nodes 150 support multiplefilesystems contemporaneously. Combining embodiments of the storagenodes 150 with redundant remote procedure call caches and the supportfor multiple filesystems offers fault-tolerant operation under multiplefilesystems. It should be appreciated that a remote procedure callrefers to a technology that allows a computer program to cause asubroutine or procedure to execute in another address space, commonly onanother computing device on a shared network, without a programmerexplicitly coding the details for this remote interaction in someembodiments.

The storage system depicted in FIG. 4, and variations thereof, supportsmultiple interfaces, which can be file or object-oriented interfacesincluding, for example, NFS 3 (network file system three), NFS 4(network file system four), CIFS (common Internet file system), ObjectAPI (object application programming interface), and iSCSI (Internetsmall computer system interface). In a cluster environment, each objecthas operatives, which could be based on the inode level, whichrepresents a file or an object with an operative. Most filesystems arehierarchical, with directories and name spaces, and filenames withindirectories. Each filename is represented internally with an inodenumber.

In contrast with embodiments employing flash memory or other solid-statememory, spinning media such as hard disks, optical disks and floppydisks tend to cluster data sequentially or close together on the disk,which allows for rapid retrieval or retrieval in parallel. Directoriesand inodes are often placed close together. Adding fields to data ormetadata tends to be difficult, and so is typically not done. With flashor other solid-state memory, there is no such requirement. With spinningmedia, to upgrade to change a storage scheme per a new filesystem, thedata is typically offloaded and then reloaded back onto the system underthe schema of the new filesystem. With flash or other solid-state memoryon present embodiments, new data can be stored using a new scheme,without having to offload old data and rewrite the old data back on. Olddata can remain under the old filesystem. For example, in presentembodiments of a storage system, an inode version two and an inodeversion three can coexist in memory, with support for both. As a furtherexample, files under both versions could not only be maintained butcould also remain online during a conversion, for example from filesunder version two to version three. A storage cluster 160, having agroup of storage nodes 150, can serve as a store for multiplefilesystems. This contrasts with traditional storage, where a filesystemwould be dedicated to a specified set of storage nodes or servers. Insome embodiments of the storage system, the entire storage cluster 160is available for storage to each filesystem, or a subset of the entirestorage cluster 160, i.e., a specified set of storage nodes 150 could bededicated to a specified filesystem.

As an example of how two (or more) differing inode versions,corresponding to two different filesystems, could coexist in anembodiment of the storage system, a top of rack switch 350 could routeremote procedure calls according to inode version. The storage systemdepicted in FIG. 4 includes a top of rack switch 350 and multiplestorage clusters 160, each having one or more storage nodes 150. Forexample, the access from outside of the storage system is through anetwork such as ethernet, which connects to the top of rack switch 350.In one embodiment, the top of rack switch 350 can determine where theauthority is located for a given inode number, and send the request foraccess (e.g., the remote procedure call) to that non-volatilesolid-state storage 152 (and corresponding storage node 150) having thatauthority. This could be accomplished through remote procedure callmessage parsing, and use of a table such as a lookup table or a hashtable. A remote procedure call has data encoded in a specified format,and the top of rack switch 350 determines the inode number according tothe message parsing, and sends the remote procedure call to theauthority responsible for that inode number. In an embodiment of the topof rack switch 350 that lacks this capability, the top of rack switchcould send the remote procedure call message down to one of the storageclusters 160, i.e., to one of the storage nodes 150 and accompanyingnon-volatile solid-state storage 152. The storage node 150 ornon-volatile solid-state storage 152 would then determine whichnon-volatile solid-state storage 152 has the authority, and forward theremote procedure call message to that non-volatile solid-state storage152. In FIG. 4 the arrow from the top of rack switch 350 to one of thestorage clusters 160, and another arrow from that storage cluster 160 tothe bottom-most storage cluster 160 illustrates this forwarding ofremote procedure call messages. In case a storage node or non-volatilesolid-state storage unit is unreachable, the storage nodes 150 ornon-volatile solid-state storages 152 could determine a replacementauthority and forward the remote procedure call message to thatnon-volatile solid-state storage 152. This operation is depicted in FIG.4 by another arrow from the top of rack switch 350 to one of the storageclusters 160, and an arrow from that storage cluster 160 to thebottom-most storage cluster 160. Whichever authority finally handlesdata (whether a write or a read, etc.) returns the reply as a directserver response (DSR) in some embodiments, depicted as an arrow from thebottommost storage cluster 160 to the top of rack switch 350. Someembodiments do not perform a DSR, and may instead forward responsesthrough one or more storage nodes 150. In further embodiments, othertypes of switches, or other locations for a switch are readily devised.

As a further example of how two or more differing inode versions,corresponding to two or more different filesystems, could coexist in anembodiment of the storage system, various Internet Protocol addresshandling schemes may be utilized. Files with one inode version,corresponding to one filesystem, could be sent to and read from oneInternet Protocol address, and files with another inode version,corresponding to another filesystem, could be sent to and read fromanother Internet Protocol address. Internet Protocol address handlingcould be performed according to numerous mechanisms, some of which aresummarized below.

As a first mechanism for Internet Protocol address handling, oneInternet Protocol address could be used for the entire cluster, and thetop of rack switch 350 could look in each packet and determine, frominformation in the header, an inode version. From the inode version, thetop of rack switch 350 could apply a hash calculation, range-basedassignment table, or a lookup table, and determine which storage node150 handles that data. The top of rack switch 350 could then send theremote procedure call to the storage node 150 having the authority forthat data, eliminating the need for forwarding the remote procedure callfrom one storage node 150 to another storage node 150. As a secondmechanism for Internet Protocol address handling, the remote procedurecall could be sent randomly to one of the storage nodes 150. Thatstorage node 150 could then determine which storage node 150 hasauthority for the data, according to the inode number. The storage node150 could then forward the remote procedure call accordingly. As a thirdmechanism for Internet Protocol address handling, differing InternetProtocol addresses could be used with one storage cluster 160, one groupof storage clusters 160, or one portion of a storage cluster 160handling one Internet Protocol address or several Internet Protocoladdresses. Another storage cluster 160, group of storage clusters 160 orportion of a storage cluster 160 handling another Internet Protocoladdress or several Internet Protocol addresses. Files under onefilesystem could have one address, and files under another filesystemcould have another address.

In the above scenarios, the client could be operating over InternetProtocol, which may or may not be reliable. As an example, the clientmay retransmit a remote procedure call, but, meanwhile, the filesystemmight have actually responded, which can lead to inconsistency (e.g. dueto multiple executions of the same remote procedure call). A measure offault tolerance, employing redundant remote procedure call caches canmitigate these potential problems, as described below.

FIG. 5 is a block diagram of storage nodes 150 with redundant remoteprocedure call caches 354, 356 in accordance with some embodiments. Eachstorage node 150 has an authority assignment table 352, a remoteprocedure call cache 354, and one or more mirrored remote procedure callcaches 356. The remote procedure call cache 354 is located where theauthority is located in order to minimize risk of breakage in acommunication path between the location of the authority and thelocation of the remote procedure call cache 354 in some embodiments.Locating the remote procedure call cache 354 distal to the authority ispossible but may increase delays and increase risk of such breakage of acommunication path.

In various embodiments, the remote procedure call cache 354 and mirroredremote procedure call cache 356 are implemented in the memory 154coupled to the CPU 156 of a storage node 150 (see FIG. 1) or in thenon-volatile random access memory 204 of the non-volatile solid-statestorage 152 (see FIG. 3). In other embodiments, the remote procedurecall cache 354 and mirrored remote procedure call cache 356 areimplemented in the dynamic random access memory 216 coupled to thecontroller 212 in the non-volatile solid-state storage 152, in the flash206 in the non-volatile solid-state storage 152, or in flash memory on astorage node 150. In one embodiment, the remote procedure call cache 354and one or more mirrored remote procedure call caches 356 areimplemented as metadata 230 in the non-volatile random access memory 204of the non-volatile solid-state storage 152. In operation, a storagenode 150 minors the remote procedure call cache 354 in at least oneother storage node 150 of the storage cluster.

When a remote procedure call arrives for servicing, the storage node 150or the non-volatile solid-state storage 152 determines whether theremote procedure call has already been serviced. This can beaccomplished by checking the remote procedure call cache 354 to see if aresult is already posted, i.e., the result of servicing the remoteprocedure call is available. For example, the result could be anacknowledgment that a data write or update to a directory structure hastaken place, or the result could be error corrected data from a dataread. If a result has been posted, the result is returned as a responseto the remote procedure call, but the servicing internal to the storagenode 150 or the non-volatile solid-state storage 152 is not repeated. Inthis manner, a repeated remote procedure call can be answered withoutcausing inconsistency in the storage node 150 and/or non-volatilesolid-state storage 152.

In a case where the remote procedure call cache 354 is unreachable, oneor more of the remaining storage nodes 150 or non-volatile solid-statestorages 152 locates the corresponding mirrored remote procedure callcache 356 and determines whether a result of servicing the remoteprocedure call is already posted. This scenario could occur , forexample, if the non-volatile solid-state storage 152 or the storage node150 having the remote procedure call cache 354 is unresponsive orotherwise unreachable. The result, if available from the mirrored remoteprocedure call cache 356, is then returned as above. If there is noresult, from either the remote procedure call cache 354 or the mirroredremote procedure call cache 356 as appropriate, the remote procedurecall is serviced and responded to with the result of that service.

In one embodiment, each storage node 150 mirrors the remote procedurecall cache 354 in two other storage nodes 150, as depicted in FIG. 5.For example, the leftmost storage node 150 could send a copy of thecontents of the remote procedure call cache 354 to two other storagenodes 150. Each of these other storage nodes 150 would place the copiedcontents of the remote procedure call cache 354 in a mirrored remoteprocedure call cache 356 of that storage node 150 as depicted by thearrows in FIG. 5. Each remote procedure call message has a uniquetransaction identifier, signed by the client, uniquely identifying thetransaction. If a storage node 150 is unreachable (whether permanentlyor temporarily), a copy of the remote procedure call cache 354 isavailable in at least one other storage node 150, e.g., in a mirroredremote procedure call cache 356. Each remote procedure call cache 354,and mirrored remote procedure call cache 356, contains the transactionidentifier, the client identifier, and the result (e.g., an indicationof whether or not the action is complete), in one embodiment. The remoteprocedure call, and information relating thereto as stored in the remoteprocedure call cache 354, are forms of metadata in some embodiments.

In one embodiment, each storage node 150 consults the table 352 residentin that storage node 150. For example, table 352 could reside in thememory of the storage node 150, or the memory of a non-volatilesolid-state storage 152 of the storage node 150, and so on. The remoteprocedure call cache 354 is maintained for an authority for which thatstorage node 150 has primary authority, as indicated in table 352. Thestorage node 150 minors the remote procedure call cache 354, by sendingupdates (copies) of the contents of the remote procedure call cache 354to the storage node 150 identified as having the first backup authority,and to the storage node 150 identified as having the second backupauthority, according to the table 352. In further embodiments,additional copies of the remote procedure call cache 354 could bemirrored, or the copies and mirror caches could be distributed in adifferent manner. Locations of the mirrored remote procedure call caches356 could be tracked by another table or tracking mechanism instead oftable 352 in other embodiments.

In a scenario where a mirrored remote procedure call cache 356 belongingto a non-volatile solid-state storage 152 or a storage node 150 isunreachable (e.g., if the solid-state storage 152 or non-volatilestorage node 150 itself is unreachable), the remaining storage nodes 150can determine and assign a replacement mirrored remote procedure callcache 356. The determination and assignment of a replacement mirroredremote procedure call cache 356 may include applying one or moremechanisms, such as witnessing, voting, volunteering, consulting thetable 352 to find a backup authority and assigning the mirrored remoteprocedure call cache 356 to the same node as has the backup authority,and so on. Or, a mirrored remote procedure call cache 356 could beassigned to a differing node than the node having the backup authority.Once the replacement mirrored remote procedure call cache 356 isdetermined, the storage node 150 corresponding to that replacementmirrored remote procedure call cache 356 can mirror the remote procedurecall cache 354 to the mirrored remote procedure call cache 356. Thiswould be followed by servicing the remote procedure call, and respondingto the remote procedure call with a result. In some embodiments, remoteprocedure call entries to the remote procedure call cache 354 and/or themirrored remote procedure call cache(s) 356 are deleted after a timeexpires. This can be accomplished using one or more timers, ortimestamps, etc.

FIG. 6 is a flow diagram of a method for caching a remote procedure callin a storage cluster, which can be practiced on or by embodiments of thestorage cluster, storage nodes and/or non-volatile solid-state storagesdisclosed herein. Many of the actions described in the method can beperformed by one or more processors, such as processors on storage nodesand processors in non-volatile solid-state storages in some embodiments.In an action 602, a remote procedure call is sent from a top of rackswitch (or other type or location of a switch) to a storage cluster. Insome embodiments, the top of rack switch performs parsing of the remoteprocedure call, and sends the remote procedure call directly to thestorage cluster according to the parsing (and thus action 606 would notbe needed). In other embodiments a storage cluster or storage node maydetermine an inode number for data referenced in the remote procedurecall, and then determine which storage node is the appropriatedestination for the remote procedure call.

In an action 604, the remote procedure call is received at a storagenode. It should be appreciated that if the receiving storage node is thecorrect storage node for the remote procedure call, in accordance withthe authority of the storage node, the receiving storage node need notperform action 606. In an action 606, the remote procedure call isforwarded to a destination storage node i.e., to the storage node whichis the appropriate destination for the remote procedure call. The remoteprocedure call cache belonging to this (destination) storage node ischecked to see if there is already a result from executing the remoteprocedure call, in a decision action 608. This would be the case if aremote procedure call was previously executed, but the client sent aduplicate remote procedure call, e.g., if the result got lost en routeor was excessively delayed. If the answer is yes, there is a result fromexecuting the remote procedure call, flow branches to the action 620, inorder to respond to the remote procedure call with the result. If theanswer is no, there is no result yet, flow continues to the action 610in order to write to the remote procedure call cache and then servicethe remote procedure call.

In the action 610, the remote procedure call, or information relating tothe remote procedure call, is written to a remote procedure call cache.This is the remote procedure call cache belonging to the destinationstorage node. The method proceeds to action 612, where the remoteprocedure call cache is mirrored to one or more mirrored remoteprocedure call caches. The mirrored remote procedure call cache is onanother one of the storage nodes, as described above with reference toFIGS. 4 and 5. The level of redundancy for the remote procedure call maybe set according to policies for a storage node in some embodiments.

In a decision action 614, it is determined if the mirrored remoteprocedure call cache is unreachable. If the answer is no, the mirroredremote procedure call cache is reachable, flow branches to the action618, in which the remote procedure call is serviced. In some embodimentsthe remote procedure call is serviced after the level of redundancy forthe remote procedure call is achieved. After servicing the remoteprocedure call, the storage node or solid-state storage responds to theremote procedure call with the result, in an action 620. The flow thenproceeds back to the action 602, for the next remote procedure call andrepeats as described above. If the answer to the decision action 614 isyes, the mirrored remote procedure call cache is unreachable, flowbranches to the action 616. In the action 616, a replacement minor,i.e., mirrored remote procedure call cache, is determined. In someembodiments, a prerequisite to action 616 is to determine that the levelof redundancy for the remote procedure call has been achieved.Determination of a replacement remote procedure call cache can beperformed via application of a table, as described above in someembodiments. After the action 616, flow continues back to the action612, in order to minor the remote procedure call cache to the newlydetermined replacement minor and then service the remote procedure callin the action 618.

It should be appreciated that the methods described herein may beperformed with a digital processing system, such as a conventional,general-purpose computer system. Special purpose computers, which aredesigned or programmed to perform only one function may be used in thealternative. FIG. 7 is an illustration showing an exemplary computingdevice which may implement the embodiments described herein. Thecomputing device of FIG. 7 may be used to perform embodiments of thefunctionality for a storage node or a non-volatile solid-state storagein accordance with some embodiments. The computing device includes acentral processing unit (CPU) 701, which is coupled through a bus 705 toa memory 703, and mass storage device 707. Mass storage device 707represents a persistent data storage device such as a disc drive, whichmay be local or remote in some embodiments. The mass storage device 707could implement a backup storage, in some embodiments. Memory 703 mayinclude read only memory, random access memory, etc. Applicationsresident on the computing device may be stored on or accessed via acomputer readable medium such as memory 703 or mass storage device 707in some embodiments. Applications may also be in the form of modulatedelectronic signals modulated accessed via a network modem or othernetwork interface of the computing device. It should be appreciated thatCPU 701 may be embodied in a general-purpose processor, a specialpurpose processor, or a specially programmed logic device in someembodiments.

Display 711 is in communication with CPU 701, memory 703, and massstorage device 707, through bus 705. Display 711 is configured todisplay any visualization tools or reports associated with the systemdescribed herein. Input/output device 709 is coupled to bus 705 in orderto communicate information in command selections to CPU 701. It shouldbe appreciated that data to and from external devices may becommunicated through the input/output device 709. CPU 701 can be definedto execute the functionality described herein to enable thefunctionality described with reference to FIGS. 1-6. The code embodyingthis functionality may be stored within memory 703 or mass storagedevice 707 for execution by a processor such as CPU 701 in someembodiments. The operating system on the computing device may beMS-WINDOWS™, UNIX™, LINUX™, iOS™, CentOS™, Android™, Redhat Linux198 ,z/OS™, or other known operating systems. It should be appreciated thatthe embodiments described herein may be integrated with virtualizedcomputing system also.

Detailed illustrative embodiments are disclosed herein. However,specific functional details disclosed herein are merely representativefor purposes of describing embodiments. Embodiments may, however, beembodied in many alternate forms and should not be construed as limitedto only the embodiments set forth herein.

It should be understood that although the terms first, second, etc. maybe used herein to describe various steps or calculations, these steps orcalculations should not be limited by these terms. These terms are onlyused to distinguish one step or calculation from another. For example, afirst calculation could be termed a second calculation, and, similarly,a second step could be termed a first step, without departing from thescope of this disclosure. As used herein, the term “and/or” and the “/”symbol includes any and all combinations of one or more of theassociated listed items.

As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”,“comprising”, “includes”, and/or “including”, when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. Therefore, the terminology usedherein is for the purpose of describing particular embodiments only andis not intended to be limiting.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

With the above embodiments in mind, it should be understood that theembodiments might employ various computer-implemented operationsinvolving data stored in computer systems. These operations are thoserequiring physical manipulation of physical quantities. Usually, thoughnot necessarily, these quantities take the form of electrical ormagnetic signals capable of being stored, transferred, combined,compared, and otherwise manipulated. Further, the manipulationsperformed are often referred to in terms, such as producing,identifying, determining, or comparing. Any of the operations describedherein that form part of the embodiments are useful machine operations.The embodiments also relate to a device or an apparatus for performingthese operations. The apparatus can be specially constructed for therequired purpose, or the apparatus can be a general-purpose computerselectively activated or configured by a computer program stored in thecomputer. In particular, various general-purpose machines can be usedwith computer programs written in accordance with the teachings herein,or it may be more convenient to construct a more specialized apparatusto perform the required operations.

A module, an application, a layer, an agent or other method-operableentity could be implemented as hardware, firmware, or a processorexecuting software, or combinations thereof. It should be appreciatedthat, where a software-based embodiment is disclosed herein, thesoftware can be embodied in a physical machine such as a controller. Forexample, a controller could include a first module and a second module.A controller could be configured to perform various actions, e.g., of amethod, an application, a layer or an agent.

The embodiments can also be embodied as computer readable code on anon-transitory computer readable medium. The computer readable medium isany data storage device that can store data, which can be thereafterread by a computer system. Examples of the computer readable mediuminclude hard drives, network attached storage (NAS), read-only memory,random-access memory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes, and otheroptical and non-optical data storage devices. The computer readablemedium can also be distributed over a network coupled computer system sothat the computer readable code is stored and executed in a distributedfashion. Embodiments described herein may be practiced with variouscomputer system configurations including hand-held devices, tablets,microprocessor systems, microprocessor-based or programmable consumerelectronics, minicomputers, mainframe computers and the like. Theembodiments can also be practiced in distributed computing environmentswhere tasks are performed by remote processing devices that are linkedthrough a wire-based or wireless network.

Although the method operations were described in a specific order, itshould be understood that other operations may be performed in betweendescribed operations, described operations may be adjusted so that theyoccur at slightly different times or the described operations may bedistributed in a system which allows the occurrence of the processingoperations at various intervals associated with the processing.

In various embodiments, one or more portions of the methods andmechanisms described herein may form part of a cloud-computingenvironment. In such embodiments, resources may be provided over theInternet as services according to one or more various models. Suchmodels may include Infrastructure as a Service (IaaS), Platform as aService (PaaS), and Software as a Service (SaaS). In IaaS, computerinfrastructure is delivered as a service. In such a case, the computingequipment is generally owned and operated by the service provider. Inthe PaaS model, software tools and underlying equipment used bydevelopers to develop software solutions may be provided as a serviceand hosted by the service provider. SaaS typically includes a serviceprovider licensing software as a service on demand. The service providermay host the software, or may deploy the software to a customer for agiven period of time. Numerous combinations of the above models arepossible and are contemplated.

Various units, circuits, or other components may be described or claimedas “configured to” perform a task or tasks. In such contexts, the phrase“configured to” is used to connote structure by indicating that theunits/circuits/components include structure (e.g., circuitry) thatperforms the task or tasks during operation. As such, theunit/circuit/component can be said to be configured to perform the taskeven when the specified unit/circuit/component is not currentlyoperational (e.g., is not on). The units/circuits/components used withthe “configured to” language include hardware—for example, circuits,memory storing program instructions executable to implement theoperation, etc. Reciting that a unit/circuit/component is “configuredto” perform one or more tasks is expressly intended not to invoke 35U.S.C. 112, sixth paragraph, for that unit/circuit/component.Additionally, “configured to” can include generic structure (e.g.,generic circuitry) that is manipulated by software and/or firmware(e.g., an FPGA or a general-purpose processor executing software) tooperate in manner that is capable of performing the task(s) at issue.“Configured to” may also include adapting a manufacturing process (e.g.,a semiconductor fabrication facility) to fabricate devices (e.g.,integrated circuits) that are adapted to implement or perform one ormore tasks.

The foregoing description, for the purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the embodiments and its practical applications, to therebyenable others skilled in the art to best utilize the embodiments andvarious modifications as may be suited to the particular usecontemplated. Accordingly, the present embodiments are to be consideredas illustrative and not restrictive, and the invention is not to belimited to the details given herein, but may be modified within thescope and equivalents of the appended claims.

What is claimed is:
 1. A plurality of storage nodes, comprising: each ofthe plurality of storage nodes having non-volatile solid-state memoryfor user data storage, a remote procedure call cache, and a mirroredremote procedure call cache configured to minor a remote procedure callcache of another storage node.
 2. The plurality of storage nodes ofclaim 1, further comprising: each of the plurality of storage nodeshaving a table, configured to indicate a primary authority, a firstbackup authority, and a second backup authority, wherein the remoteprocedure call cache corresponds to the primary authority.
 3. Theplurality of storage nodes of claim 1, wherein each of the plurality ofstorage nodes is configured to send a copy of contents of the remoteprocedure call cache to a further storage node for storage in themirrored remote procedure call cache of the further storage node.
 4. Theplurality of storage nodes of claim 1, further comprising: each of theplurality of storage nodes configured to check the remote procedure callcache and to determine whether a result of a remote procedure call isposted.
 5. The plurality of storage nodes of claim 1, furthercomprising: each of the plurality of storage nodes having a non-volatilerandom access memory (NVRAM) containing the remote procedure call cacheand the mirrored remote procedure call cache.
 6. The plurality ofstorage nodes of claim 1, further comprising: the remote procedure callcache configured to store, as metadata relating to a remote procedurecall, a transaction identifier, a client identifier, and a result. 7.The plurality of storage nodes of claim 1, further comprising: each ofthe plurality of storage nodes configured to determine whether amirrored remote procedure call cache is unreachable and to mirror theremote procedure call cache to a replacement mirrored remote procedurecall cache responsive to the remote procedure call cache beingunreachable.
 8. A storage cluster, comprising: a plurality of storagenodes; each storage node of the plurality of storage nodes havingnon-volatile solid-state memory for storage of user data and configuredto access data according to remote procedure calls, a remote procedurecall cache, and a mirrored remote procedure call cache; and each storagenode of the plurality of storage nodes configured to mirror the remoteprocedure call cache in the mirrored remote procedure call cache of adiffering one of the plurality of storage nodes.
 9. The storage clusterof claim 8, further comprising: each storage node of the plurality ofstorage nodes configured to mirror, in the mirrored remote procedurecall cache of the each storage node, the remote procedure call cache ofa further one of the plurality of storage nodes.
 10. The storage clusterof claim 8, further comprising: a switch configured to parse the remoteprocedure calls and to route the remote procedure calls to the pluralityof storage nodes.
 11. The storage cluster of claim 8, furthercomprising: each of the plurality of storage nodes having a tableconfigured to indicate which storage node of the plurality of storagenodes has the remote procedure call cache corresponding to a specifieddata; and each storage node configured to forward a remote procedurecall to one of the plurality of storage nodes in accordance with thetable.
 12. The storage cluster of claim 8, further comprising: each ofthe plurality of storage nodes configured to determine whether a resultof a remote procedure call is available in the remote procedure callcache, and to respond to the remote procedure call with the result in anevent that the result is available in the remote procedure call cache.13. The storage cluster of claim 8, further comprising: the plurality ofstorage nodes configured to determine a replacement mirrored remoteprocedure call cache responsive to the remote procedure call cache of aone of the plurality of storage nodes being unreachable, wherein theplurality of storage nodes obtains a level of redundancy of the remoteprocedure call across the plurality of storage nodes.
 14. A method ofoperating a remote procedure call cache in a storage cluster having aplurality of storage nodes, comprising: receiving a remote procedurecall at a first one of the plurality of storage nodes; writinginformation, relating to the remote procedure call, to a remoteprocedure call cache of the first one of the plurality of storage nodes;mirroring the remote procedure call cache of the first one of theplurality of storage nodes in a mirrored remote procedure call cache ofa second one of the plurality of storage nodes.
 15. The method of claim14, further comprising: mirroring the remote procedure call cache of thefirst one of the plurality of storage nodes in a mirrored remoteprocedure call cache of a third one of the plurality of storage nodes.16. The method of claim 14, further comprising: mirroring remoteprocedure call caches of two further storage nodes in two mirroredremote procedure call caches of the first one of the plurality ofstorage nodes.
 17. The method of claim 14, further comprising:determining a replacement mirrored remote procedure call cache, inaccordance with a table, responsive to the mirrored remote procedurecall cache of the second one of the plurality of storage nodes beingunreachable; mirroring the remote procedure call cache of the first oneof the plurality of storage nodes to the replacement mirrored remoteprocedure call cache.
 18. The method of claim 14, further comprising:routing the remote procedure call to the first one of the plurality ofstorage nodes in accordance with the first one of the plurality ofstorage nodes having authority for files in a first filesystem, whereinthe second one of the plurality of storage nodes has authority for filesin a second filesystem, the first filesystem differing from the secondfilesystem.
 19. The method of claim 14, further comprising: routing theremote procedure call from a switch to the first one of the plurality ofstorage nodes, based on a first inode number; and routing a secondremote procedure call from the switch to the second one of the pluralityof storage nodes, based on a second inode number.
 20. The method ofclaim 14, further comprising: receiving a further remote procedure callat the first one of the plurality of storage nodes; checking the remoteprocedure call cache of the first one of the plurality of storage nodesas to whether a result of the remote procedure call is available; andresponding to the remote procedure call with the result, responsive to adetermination that the result of the remote procedure call is available.